I planted about 20-30 trees and shrubs only a couple of weeks ago and this wet weather will settle them in nicely.

Depending on how big they are remember to stake them to keep them firm in the ground but stake them low on the trunk so that the top of the tree can still move and just the very bottom of the trunk remains firmly in place.

I grow them in my wildflower area but as you say they don't last long. An alternative which might be worth a go I know as Pheasant's Eye? These are bright red and flower for longer than poppies during June and July. I had one just pop up in the garden a couple of years ago and by dead-heading kept it going for quite some time.

Verdun, that sounds lovely and great to keep the interest going throughout the summer months!

Arabis is a good suggestion and the one I had in my old garden gave off a lovely scent.

The other thing that I have grown on the dry parts of my rockery are Californian Poppy, they would like the dry conditions and add a lovely splash of bright orange although you can get pink and cream varieties also. Only annual but might be worth buying a packet of seed and sprinkling over the gaps in the wall and see what grows!?...

There is such a long list of wildlife friendly plants, trees etc and over the last few years there have been lots of extremely good and informative threads written on the RSPB Community Forum's 'Greenfingers' section. Basically there you will find everything is about wildlife gardening and planting for all sorts of wildlife and native shrubs, hedges etc.

Take a look by following this link and then browse this thread and the whole 'Greenfingers' section, I guarantee you will find what you're looking for there!